KH2PO4 problem from Dr Mallick?

[PeterGwee]

Hi guys, would like to know any of you are using the KH2PO4 powder aka potassium dihydrogen phosphate from Dr Mallick. I was pondering over why my new sera PO4 test kit could not detect the PO4 source from the powder again after I did a test today. Hence, I went off to buy the Seachem phosphorus solution (expensive ah..)..and did a test against the test kit....to my amazement, the test kit works fine..it turn dark blue after I dose alot into the test cylinder...ahhhh...something is wrong with my KH2PO4 powder!! Anybody had the same problem?

[bclee]

My suspicion is the pH and the relation to

H2PO3- <---> H+ + HPO3-- <---> 2H+ + PO3---

Just a wild guess.

BC

[PeterGwee]

Kekekeke...BC...Its been a while since I touch on chemical equations...can you please enlighten me..[] ...Eh..can my KH2PO4 powder still provide PO4 source or do I have to use the Seachem PO4 stuff from now on? I did not get any change in colouration from the PO4 test kit even if I dump a small spoonful of the powder into it!!..It cannot be?

[bclee]

Someone one the AB was mentioning that they could not get a reading with fleet enema too. Fleet enema is NaH2PO4/Na2HPO4.

I do not exactly know why... but just a hypothesis. Got to learn how the test kit work.

The phosphates can be used by plants... no problem. KH2PO4 is a common fertiliser component.

BC

[theodore]

Just a wild thought : What if the testkit can only measure free PO4 but not H2PO4 and HPO4 (i.e. when PO4 is still bound to H)?

Seachem mentioned the use of Potassium Phosphate in their product description but I don't know if it is the same type we can get from Dr. Mallick - KH2PO4 (monobasic), K2HPO4 (dibasic) and K3PO4 (tribasic).

I need a refresher course on Chemistry. Can some1 PM me some gd links to read? [:] [:] [:]

[theodore]

Peter,

To add onto my wild thought : How abt adding a base (e.g. NaOH) to ur KH2PO4 solution before testing?

2OH(-) + H2PO4(-) --> 2H20 + PO4(3-)

Can any chemist, teacher, student or knowledgable hobbyist verify/dispute this?

[bclee]

I am guessing that you need to acidify the solution more. If the Sera PO4 kit is using the ascorbic acid method. I am guessing the reagent 1 is an acid to acidify the solution. You might want to try to add more reagent 1. Another wild guess.

Hope I am not misleading anyone... just guessing only.

BC

[scang7]

hope this is correct...

KH2PO4 will dissociate to give K+, 2H+ and PO4(3-)... the lack of PO4(3-) could be due to the high concentration of H+ that shift the eqm to the left which give KH2PO4. Therefore little or no PO4(3-) is detected.

hope i am rite

[CK Yeo]

Cannot be rite?

A few possibilities. Can you describe how you test the KH2PO4? Could it be that you forgot to add something? Could you have mixed up K2SO4 with KH2PO4?

ck

[PeterGwee]

Yeo...I followed the exact instruction on the Sera PO4 test kit..no way it is wrong? I read the label on the KH2PO4...so many times already!!! Unless Dr Mallick labelled it wrongly?

[DEA]

scang7 seems to have hit it dead on
in water, KH2PO4 gives K+ and H2PO4-

H2PO4- + H2O <--> HPO4-- + H3o+ pKa 7.21
HPO4-- + H2O <--> PO4--- + H3o+ pKa 12.7

thus the dibasic in the name
you might see the reversible sign in there
with a high concentration of H30+ ions in the water, the equilibrium will shift to the left and you will have no free phosphate ions, which SHOULD be what the test kits measure

to get the full phosphate reading you'll probably need to add base to make the pH >12.7 if iirc

[PeterGwee]

No reading is fine...just want to know if plants can extract the phosphate from the KH2PO4..?

[DEA]

as previously mentioned by bclee, plants have no problems with it
biological systems should be able to extract the phosphate

[PeterGwee]

Okie..thanks.. Just wanna confirm thats all.

[Nick]

So, what's the use of a PO4 test kit? Is it necessary?

[PeterGwee]

Nick, it is used to detect presence of phosphate in the water column. Phosphate is a macro nutrient required for plants.

[Nick]

errhh... that's not what I mean. I know about PO4 stuff, but since the test kit cannot even detect PO4, aren't we better off not buying?

[PeterGwee]

That depends on how the test kit works...Now it seems it does not detect any available PO4 with KH2PO4 from Dr Mallick...but it does detect the PO4 from Seachem phosphorus. Its up to you whether you find it important to test or not. Actually Tom Barr estimative dosing works...you just dose to make sure that all nutrients are available and high but not to the extent that it is toxic to the inhibitants by doing large weekly water changes. Not much testing except for CO2..using pH and KH to judge.

One important thing is also the amount of nitrate and phosphate available from the tap...I wrote to PUB but could not get an answer from them on a precise nitrate and phosphate level. The PUB site states a nitrate of 0.5-15ppm for my area and PO4 of less than 0.10ppm.

[Plantbrain]

I think most test kits are good for presence absense of PO4.
I use a Hach and Lamott PO4 kits. But I seldom ever use them any longer.

Both KH2PO4 and the Seachem stuff are one in the same. There should be no difference.
Greg make it from the same stuff.

Mono basic is the cheapest stuff, which is why they all use it.

Regards,
Tom Barr

[taz]

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On 4/11/2003 12:08:21 PM

Hi guys, would like to know any of you are using the KH2PO4 powder aka potassium dihydrogen phosphate from Dr Mallick. I was pondering over why my new sera PO4 test kit could not detect the PO4 source from the powder again after I did a test today. Hence, I went off to buy the Seachem phosphorus solution (expensive ah..)..and did a test against the test kit....to my amazement, the test kit works fine..it turn dark blue after I dose alot into the test cylinder...ahhhh...something is wrong with my KH2PO4 powder!! Anybody had the same problem?
----------------

I'm using similar the KH2PO4 from Dr Mallick and did a test to it. It turn beyond dirty blue on Sera test kit. BTW, I know its working cause my PO4 drop 0.25ppm per day. Now I'm dosing it daily to keep it at 0.5ppm.